This invention relates to wafer bonded vertical cavity surface emitting laser (VCSEL) systems and methods of making the same.
A VCSEL is a laser device formed from an optically active semiconductor layer (e.g., AlInGaAs or InGaAsP) that is sandwiched between a pair of highly reflective mirror stacks, which may be formed from layers of metallic material, dielectric material or epitaxially-grown semiconductor material. Typically, one of the mirror stacks is made less reflective than the other so that a portion of the coherent light that builds in a resonating cavity formed in the optically active semiconductor layer between the mirror stacks may be emitted from the device. Typically, a VCSEL emits laser light from the top or bottom surface of the resonating cavity with a relatively small beam divergence. VCSELs may be arranged in singlets, one-dimensional or two-dimensional arrays, tested on wafer, and incorporated easily into an optical transceiver module that may be coupled to a fiber optic cable.
In general, a wafer bonding technique may be characterized as a direct wafer bonding technique or a metallic wafer bonding technique. In direct wafer bonding, two wafers are fused together by mass transport at a bonding interface. Direct wafer bonding may be performed between any combination of semiconductor, oxide, and dielectric materials. Direct wafer bonding typically is performed at high temperature and under uniaxial pressure. In metallic wafer bonding, two substrates are bonded together by a metallic layer that is melted and re-solidified at a bonding interface.
Wafer bonding techniques have been used in the fabrication of optoelectronic devices. For example, U.S. Pat. No. 6,320,206 has proposed a scheme for forming optical devices having aluminum gallium indium nitride active layers and high quality mirror stacks that are wafer bonded on one or both sides of the active layers. U.S. Pat. No. 5,837,561 describes a vertical cavity surface emitting laser that is wafer bonded to a transparent substrate. A top circular metal contact is disposed on the transparent substrate and a second metal contact is disposed over the bottom mirror of the vertical cavity surface emitting laser. The transparent substrate serves as an escape medium for laser emission through the top circular metal contact. This configuration allows the heat producing active layer of the vertical cavity surface emitting laser to be mounted near a heat sink, thereby improving the performance of the device.
The invention features vertical cavity surface emitting laser systems and methods of making the same. In particular, the invention features a vertical cavity surface emitting laser system having a bottom side that may be flip-chip mounted to a driver substrate and a top side configured to transmit light through an optically transparent substrate. By this configuration, the invention enables vertical cavity surface emitting laser systems to be packed together with a greater density and operated at greater speeds relative to, for example, wire bonded vertical cavity surface emitting laser systems. In addition, such systems may be flexibly tailored to produce light over a wide range of wavelengths. Such systems also may be efficiently packaged on a wafer scale.
In one aspect, the invention features a vertical cavity surface emitting laser (VCSEL) system, comprising a substrate, a vertical stack structure, and a pair of contacts. The substrate is optically transparent to light in a selected wavelength range. The vertical stack structure has a substantially planar top side, which is wafer bonded to the optically transparent substrate, and a bottom side. The vertical stack structure includes a top mirror, a bottom mirror, and a cavity region that is disposed between the top mirror and the bottom mirror and includes an active light generation region that is operable to generate light in the selected wavelength range. The vertical stack structure is constructed and arranged to direct light generated in the cavity region to the optically transparent substrate. First and second contacts are disposed over the bottom side of the vertical stack structure and are electrically connected for driving the cavity region.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
In some embodiments, the optically transparent substrate comprises glass (e.g., borosilicate glass). In other embodiments, the optically transparent substrate comprises gallium phosphide.
The VCSEL system may further comprise a lens that is disposed on the glass substrate in alignment with the active light generation region.
In some embodiments, at least one of the top mirror and the bottom mirror has a layer with a peripheral region that is oxidized into an electrical insulator as a result of exposure to an oxidizing agent. In these embodiments, the VCSEL system may further comprise two or more etched holes each extending from a substantially planar surface of the bottom mirror to the oxidized peripheral region.
The top mirror and the bottom mirror preferably each comprises a system of alternating layers of different refractive index materials. For example, the top mirror and the bottom mirror each may comprise a system of alternating layers of relatively high aluminum content AlGaAs and relatively low aluminum content AlGaAs.
In some embodiments, the VCSEL system further comprises an integrated circuit that is bonded to the pair of contacts and is operable to drive the cavity region.
In another aspect, the invention features a method of fabricating the above described VCSEL system. In accordance with this inventive method, a sacrificial substrate is provided. A vertical stack structure having a substantially planar top side and a bottom side is formed over the vertical stack structure. The vertical stack structure includes a top mirror, a bottom mirror, and a cavity region that is disposed between the top mirror and the bottom mirror and includes an active light generation region operable to generate light in a selected wavelength range. The vertical stack structure is constructed and arranged to direct light generated in the cavity region away from the sacrificial substrate. The substantially planar top side of the vertical stack structure is wafer bonded to a substrate that is optically transparent to light in the selected wavelength range. The sacrificial substrate is removed after the optically transparent substrate has been wafer bonded to the substantially planar top side of the vertical stack structure. First and second contacts that are electrically connected for driving the cavity region are formed over the bottom side of the vertical stack structure.